1. Technical Field
The present disclosure relates to a laser gas analysis apparatus. Particularly, it relates to a laser gas analysis apparatus which eliminates a signal transmission delay varying depending on an installation environment to thereby improve the accuracy of measurement.
2. Related Art
A laser gas analyzer using a TDLAS (Tunable Diode Laser Absorption Spectroscopy) method has an advantage in that the laser gas analyzer can measure high-temperature or the concentration of a target component to be measured such as corrosive gas or the like in a non-contact manner at a high speed in real time with a high component selectivity without being interfered by other components by only irradiating the target to be measured with laser beam from a tunable diode laser.
FIG. 3 is a block diagram showing a related-art laser gas analysis apparatus using the TDLAS method. The laser gas analysis apparatus is constituted by a light source unit including a semiconductor laser for emitting a laser beam into a process gas atmosphere, and a detection unit including a light receiving element for detecting the laser beam transmitted through a measurement space in the process gas atmosphere and an arithmetic processor for processing an output signal of the light receiving element.
In the laser gas analysis apparatus shown in FIG. 3, an optical absorption spectrum peculiar to molecules of a to-be-measured target component in the range of from an infrared region to a near-infrared region based on vibration and rotation energy transition of the molecules is measured by use of a semiconductor laser having an extremely narrow spectral linewidth of an oscillation wavelength. The most of molecules including O2, NH3, H2O, CO, CO2, etc. have molecule-specific absorption spectra in the range of from the infrared region to the near-infrared region. The concentration of a target component can be calculated when the optical absorption quantity (absorbance) in a specific wavelength is measured.
In FIG. 3, a diode laser 11 provided as a semiconductor laser in a light source unit 10 emits a laser beam into an atmosphere of process gas 20. The laser beam outputted by the diode laser 11 has an extremely narrow spectral linewidth of an oscillation wavelength. The oscillation wavelength can be changed by changing the laser temperature or the driving current. Thus, only one of absorption peaks in the absorption spectra can be measured.
Accordingly, an absorption peak which has not been affected by interference gas can be selected so that high wavelength selectivity can be obtained without being affected by other interference components. It is therefore possible to measure the process gas directly without removing the interference gas in a stage prior to the measurement.
When the oscillation wavelength of the diode laser 11 is changed near one absorption line of a component to be measured, a spectrum can be measured correctly without overlapping with any other interference component. However, the shape of the spectrum changes in accordance with a broadening phenomenon of the spectrum caused by the process gas temperature, the process gas pressure, coexisting gas components, etc. It is therefore necessary to correct the spectrum in actual process measurement accompanied by those environmental fluctuations.
To this end, the apparatus in FIG. 3 uses a spectrum area method in which while the oscillation wavelength of the diode laser 11 is changed, an absorption spectrum is measured to obtain the area of the spectrum, and the area of the spectrum is converted into component concentration.
Another laser gas analysis apparatus uses a peak height method in which a component to be measured is obtained from the height of a peak of an absorption spectrum or a 2f method in which a wavelength changing signal is modulated and the concentration of a component to be measured is obtained front a P-P (Peak to Peak) value of a waveform modulated with a frequency twice as high as the frequency of the signal. These methods are apt to be greatly affected by fluctuations of temperature, pressure, coexisting gas components, etc.
On the other hand, in principle, the spectrum area method is a method not affected by any change caused by a difference in coexisting gas components (the area of a spectrum is substantially fixed regardless of the coexisting gas components). The spectrum area method exhibits a linear change with respect to the fluctuation of pressure in principle.
In the peak height method or the 2f method, all the three fluctuation factors (temperature, pressure and coexisting gas components) have nonlinear influence. Correction is difficult when these fluctuation factors coexist. According to the spectrum area method, however, linear correction for the fluctuation of gas pressure and nonlinear correction for the fluctuation of gas temperature can be performed to achieve accurate correction.
The laser beam transmitted through the atmosphere of the process gas 20 is received by a light receiving element 31 as a constituent component of a detector circuit 40 which is provided in a detection unit 30. The received laser beam is converted into an electric signal.
An output signal of the light receiving element 31 is adjusted to a suitable amplitude level by a variable gain amplifier 32, and inputted to an converter 33, in which the resulting signal is converted into a digital signal.
In sync with change of the oscillation wavelength of the diode laser 11, output data of the A/D converter 33 are integrated in an integrator 34 and stored into a memory 35 a predetermined number of times (for example, several hundred times to several thousand times) repeatedly between the integrator 34 and the memory 35 as constituent components of a data acquisition circuit (hereinafter referred to as DAQ circuit) 41. Therefore, noise contained in the measurement signal is removed in order to smooth the data. The measurement signal is then inputted to a CPU 36.
The CPU 36 performs an arithmetic process for analysis of the concentration of the process gas etc. based on the measurement signal from which the noise has been removed. In addition, the CPU 36 also adjusts the gain of the variable gain amplifier 32 when the amplitude level of the output signal of the light receiving element 31 is not suitable as an input level to the A/D converter 33.
A timing generating circuit 42 outputs, to a laser controller 43, a change pulse signal for changing the oscillation wavelength of laser beam to be emitted from the diode laser, and outputs, to the integrator 34, a timing pulse signal for receiving an output from the A/D converter 33.
Non-Patent Document 1 has disclosed measurement principles, features and specific measurement examples of a laser gas analyzer using tunable diode laser absorption spectroscopy.